Atomic force microscopy (or AFM) is a scanning microscopy technique that was developed at the beginning of the 80s and makes it possible to achieve a resolution on the scale of individual atoms. Unlike scanning tunneling microscopy, atomic force microscopy is not limited to forming images of conductive surfaces, thereby making it suitable for insulating materials, semiconductors or even biological samples. This technique finds application in numerous fields of pure and applied research, as well as in the microelectronics industry. A general introduction to the principles of AFM is provided by the article by F. J. Giessibl and C. F. Quate “Exploring the nanoworld with atomic force microscopy”, Physics Today, December 2006, pages 44-50.
The main component of a conventional atomic force microscope is a probe that consists of a cantilever that is fixed at one end and provided at the opposite end with a tip oriented towards the surface of the sample to be observed. The length of the cantilever is generally of the order of a few tens or hundreds of micrometers, and the tip has a radius of curvature of a few tens or hundreds of nanometers. Such a probe, which generally consists of monocrystalline silicon or silicon nitride, may be fabricated by means of conventional photolithographic techniques, and at low cost. When the tip of the probe is brought close to a surface, it is subject to attractive or repulsive chemical, van der Waals, electrostatic and/or magnetic forces. By measuring these forces while the tip scans the surface of the sample to be observed, it is possible to reconstruct an image of the latter. The forces exerted between the tip and the sample may be measured in various ways. According to the oldest and simplest technique (static AFM), these are limited to observing, in particular through optical means, the deflection of the cantilever bearing the tip.
Improved sensitivity may be obtained by vibrating this cantilever in one of its natural bending modes, and by observing the variations in resonant frequency generated by the gradients of these forces (dynamic AFM). In practice, the dynamic technique is generally preferred for observations made in vacuum or in air. This technique is less suitable for observations in a liquid medium, since the vibrations of the cantilever are heavily damped thereby, which negatively affects the quality factor of the probe.
Another imaging technique consists in bringing the tip down to the surface, then retracting it. During the approach phase, the tip comes into contact with the surface, and the cantilever bearing it flexes; during the retraction phase, it adheres to the surface for a certain time, and the cantilever then flexes in the opposite direction. Since the amplitude of the movement is known, measuring the deformation of the cantilever over time makes it possible to determine the topography of the sample and its local mechanical properties, one point at a time. This technique, referred to as force curve imaging, is known for example from document US2012/0131702. It is non-resonant; thus, the displacement of the probe must occur at a much lower frequency than that of the fundamental mode of vibration of the cantilever. This limits the speed of image acquisition.
It is also known practice to employ AFM probes using planar modes of vibration—“vertical movement” is also spoken of—which make it possible to achieve very high quality factors even in dynamic AFM mode in viscous media.
For example, the article by Toshu An et al. “Atomically-resolved imaging by frequency-modulation atomic force microscopy using a quartz length-extension resonator”, Applied Physics Letters 87, 133114 (2005) describes a probe for AFM comprising a micromechanical resonator formed by a quartz beam, held in its medium by a rigid frame that is also made of quartz, which vibrates in an extensional mode. An AFM tip is bonded to one end of this beam, aligned with its longitudinal axis. This resonator exhibits a high quality factor, but also substantial rigidity which greatly limits the amplitude of the vibrations (typically smaller than 1 nm or at most a few nanometers). Additionally, the probe is not produced as one piece, thereby limiting the miniaturization thereof.
The Swiss company SPECS GmbH markets a “KolibriSensor” AFM probe based on this principle.
International application WO 2008/148951 describes a monolithic AFM probe employing a ring- or disk-shaped resonator which oscillates in a volume mode (planar deformation). Such a probe makes it possible to achieve high frequencies, which is favorable for obtaining a high quality factor even when it is used in a viscous medium. In addition, it is less stiff than the probe described by Toshu An et al., and lends itself to greater miniaturization since it can be produced as one piece. However, balancing the masses attached to the resonator—essential for guaranteeing the presence of a mode with a high quality factor—is difficult.
In his thesis “Fabrication de micro-résonateurs haute fréquence pour la microscopie à force atomique sur des objets biologiques” (“Fabrication of high-frequency micro-resonators for atomic force microscopy on biological objects”) defended at Lille University of Science and Technology on Dec. 13, 2011, B. Walter (one of the present inventors) has described an AFM probe comprising a tip attached to the median region of a flexible beam which exhibits two or four points of fixation, which points are located on either side of the tip. The latter is oriented in a direction that is perpendicular to the longitudinal axis of the beam. Producing such a probe is difficult since the mass of the tip interferes with its modes; in addition, it is sensitive to the position of the anchors, which position must be chosen before it is possible to characterize the tip.
In his thesis “Switchable Stiffness Scanning Microscope Probe”, defended at the Technical University of Darmstadt in June 2005, Clemens T. Mueller-Falcke describes a vertical AFM probe with adjustable stiffness. In this probe, the AFM tip is borne by a longitudinal beam, which is linked to a frame by a hairpin spring and an annular mechanical resonator; the frame is itself linked to an anchor by hairpin springs. An electrostatic actuator is provided between the anchor and the substrate.
Despite their structural differences, the AFM probes using planar modes of vibration known from the prior art share a certain number of drawbacks, in particular their bulk. These bulk constraints are linked to the fact that the AFM tip protrudes relatively little from the planar substrate on which the probe is produced. Stated otherwise, the tip extends from the edge of said substrate over a distance that is very small with respect to the width of the substrate (its largest dimension perpendicular to the tip), but also with respect to its thickness (its smallest dimension perpendicular to the tip). Because of this, the tip must be held substantially perpendicular to the surface of the sample being observed by AFM, which must be planar and smooth: any incline of more than a few degrees, or any irregularity in the surface of more than a few micrometers, will lead to undesirable contact between the substrate of the probe and the sample. This severely limits the possibilities when it comes to studying biological samples (which are generally not smooth) and carrying out simultaneous optical and AFM observations or analyses of one and the same region of a sample.
At first glance, it would appear possible to envisage overcoming these bulk constraints by using longer AFM tips, or tips formed at the free end of long and thin beams. However, this presents considerable difficulties. Specifically, an AFM tip or beam mounted as a cantilever and extending over a relatively substantial length (ten times its width, or even more; typically this corresponds to a few tens or hundreds of micrometers) exhibits parasitic bending modes that are liable to be activated unintentionally and to interrupt the normal operation of the beam. In addition, in the case of a probe of the type described in WO 2008/148951, an overly long—and hence heavy—tip will disrupt the elliptical deformation modes of the ring-shaped resonator.
In the case of the aforementioned article by Toshu An et al., the tip is attached to the end of the beam of considerable length (longer than 1 mm). However, in order to prevent excitation of the bending modes and to facilitate bonding of the attached tip, this beam has a relatively substantial cross section, with dimensions of the order of 100 μm, while the AFM tip protrudes from the beam only by around ten micrometers. Hence it is the longitudinal beam bearing the tip, rather than the frame supporting this beam, that introduces ultimately quite significant bulk constraints.
Document WO 2016/138398 discloses an AFM probe comprising a plurality of tips borne by longitudinal beams supported by support elements which extend beyond the edge of the substrate. These beams are thick, which limits their mechanical resonant frequency.
The invention aims to overcome the aforementioned drawbacks of the prior art. More particularly, it aims to provide a compact, vertically movable AFM probe having good mechanical properties—i.e. a planar displacement in a well-defined direction, little affected by parasitic modes within the plane or outside of the plane. Advantageously, an AFM probe according to the invention must exhibit a high resonant frequency within the plane (higher than or equal to 1 MHz) and an amplitude of displacement of the tip that may be comprised between approximately one and a hundred nanometers. According to various embodiments, it must be possible to use it in a resonant or non-resonant mode, and in particular in force curve imaging mode. In the latter case, the invention also aims to make it possible to endow the tip with a mean displacement that may reach several micrometers, in combination with a higher image acquisition rate than in the prior art.
In order to obtain a high resonant frequency (and hence image acquisition rate) without simultaneously having an overly high stiffness—and hence an insufficient amplitude of displacement of the tip—the mass of the movable elements of the probe, including the tip, must be as low as possible. This may be achieved by producing these elements on the basis of a thin-film material, having a thickness of a few hundreds of nanometers only. However, such an approach presents difficulties, since a movable portion of such low thickness necessarily exhibits a high degree of flexibility. In order to avoid parasitic deformation modes, in particular bending out of plane, it would therefore be necessary for this part to protrude from the edge of the substrate only by a few micrometers at most. This is unacceptable for at least two reasons:                First, the etching of the substrate has a tolerance of several micrometers, thereby making the position of the edge imprecisely defined. Trying too hard to miniaturize the probe therefore runs the risk of the tip not protruding from the edge of the substrate, which would render it inoperable.        Secondly, the bulk constraints would be too great.        
The invention makes it possible to overcome these obstacles by virtue of the use of a mechanical support structure anchored to the substrate, extending beyond the edge of the substrate and supporting the sensitive part of the probe without blocking it. The sensitive part of the probe may therefore be very thin, since it is the support structure which provides it with the required mechanical stability. In this way, it is possible to independently optimize the resonant frequency (which depend solely on the structure of the sensitive part, and in particular on its mass) and the mechanical stiffness with respect to bending out of plane (which mainly depends on the support structure); parasitic resonant modes are also avoided. According to one particular embodiment of the invention, the mechanical support structure is arranged “above” the sensitive part of the probe, i.e. on the side opposite the substrate.